Note: Descriptions are shown in the official language in which they were submitted.
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HDTV TRICK PLAY STREAM DERIVATION FOR VCR
This invention relates to the field of digital video
' recording, and in particular to the derivation, recording and
reproduction of MPEG like advanced television signals at non-
standard speeds.
BACKGROUND OF THE IIWENTION
A digital video cassette recorder employing a helical
scanning format has been proposed by a standardization committee.
The proposed standard specifies digital recording of standard
definition SD television signals, for example, NTSC or PAL,
and high
definition television signals having an MPEG compatible structure,
such as a proposed Grand Alliance or GA signal. The SD recorder
utilizes a compressed component video signal format employing
intra
field/frame DCT with adaptive quantization and variable length
coding. The SD digital VCR or DVCR may digitally record either
NTSC
or PAL television signals and has sufficient data recording
capability
to record an advanced television signal.
A specification of the GA signal is included in a draft
2 0 specification document titled Grand Alliance HDTV System
Specification, published in the 1994 Proceeding of the 48th
Annual
Broadcast Engineering Conference Proceedings, March 20 - 24
1994.
The GA signal employs an MPEG compatible coding method which
utilizes an intra-frame coded picture, termed I frame, a forward
2 S predicted frame, termed a P frame and a bidirectionally predicted
frame, termed a B frame. These three types of frames occur in
groups
known as GOPs or Groups Of Pictures. The number of frames in
a GOP
is user definable but may comprise, for example, 15 frames.
Each GOP
contains one I frame, which may be abutted by two B frames,
which
3 0 are followed by a P frame.
In an analog consumer VCR, "Trick Play" or TP features
such as picture in forward or reverse shuttle, fast or slow
motion, are
readily achievable, since each recorded track typically contains
one
television field. Hence, reproduction at speeds other than standard,
3 5 may result in the reproducing head, or heads, crossing multiple
tracks
and recovering recognizable picture segments. The picture segments
may be abutted and provide a recognizable and useful image.
An
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advanced television or MPEG like signal may comprise groups of
pictures or GOPs. The GOP may, for example, comprise 15 frames and ,
each frame may be recorded occupying multiple tracks on tape. For
example, if 10 tracks are allocated to each frame, then a 15 frame GOP
will comprise 150 tracks. During play speed operation, I frame data is
recovered which enables the decoding and reconstruction of the
predicted P and B frames. However, when a DVCR is operated at a non-
standard reproduction speed, the replay heads transduce sections or
segments from the multiple tracks. Unfortunately these DVCR tracks
no longer represent discrete records of consecutive image fields.
Instead these segments contain data resulting mainly from predicted
frames. However, since predicted P and B frames require preceding
data to facilitate decoding the possibility of reconstructing any usable
frames from the reproduced pieces of data is greatly diminished. In
addition the MPEG data stream is particularly unforgiving of missing or
garbled data. Thus to provide "Trick Play" or non-standard speed
replay features requires that specific data be recorded, which when
reproduced in a TP mode, is capable of image reconstruction without
the use of adjacent or preceding frame information. The specific data,
2 0 or "Trick Play" data must be semantically correct to allow MPEG
decoding. In addition, a selection of "Trick Play" speeds, may require
different TP data derivation and may require TP speed specific
recorded track locations.
To be capable of reconstruction without preceding frame
2 5 data requires that "Trick Play" specific data be derived from I frames.
The "Trick Play" specific data must be syntactically and semantically
correct to allow decoding, for example, by a GA or MPEG compatible
decoder. In addition the "Trick Play" or TP data must be inserted into
the MPEG like data stream for recording together with the normal play,
3 0 MPEG like signal. This sharing of the recording channel data capacity
rnay impose constraints in terms of TP data bit rate which may be '
provided within the available track capacity. The TP data bit rate may
be variously utilized or shared between spatial and or temporal
resolution in the derived or reconstructed TP image.
3 5 Reproduced "Trick Play" image quality may be determined
by the complexity of the TP data derivation. For example, a consumer
DVCR must derive TP data during recording, essentially in real-time
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and with only nominal additional data processing expense added
to the
DVCR cost. Thus real-time consumer DVCR "Trick Play" image quality
may appear inferior to TP image data derived by non-real time
image
processing utilizing sophisticated digital image processing.
With non-
real time TP image processing for example, an edited program
may be
processed, possibly on a scene by scene basis, possibly at non-real-time
reproduction speeds, to enable the use of sophisticated digital
image
processing techniques. Such non-real time processing may inherently
provide higher quality "Trick Play" images than that attainable
with
real time processing.
SUMMARY OF THE INVENTION:
A method for generating an MPEG compatible digital image
representative signal for recording to facilitate reproduction
at more
than one speed. The method comprises the steps of: receiving
a digital
image representative signal; temporally sub-sampling the digital
image representative signal at a rate related to a trick play
speed;
encoding the temporally sub-sampled signal to produce an MPEG
compatible trick play signal; encoding the digital image representative
data signal to produce a normal play MPEG compatible signal;
selecting
2 0 between the trick play MPEG compatible signal and the normal
play
MPEG compatible signal to produce a record formatted bit stream;
and,
recording the record formatted bit stream.
BRIEF DESCRIPTION OF THE DRAWING:
FIGURE 1 is a simplified block diagram of an inventive
2 5 system for the real-time generation of a "trick-play" data stream
having low resolution.
FIGURE 2 shows a simplified block diagram of a further
inventive system for the real-time generation of a full resolution,
"trick-play" data stream.
3 0 FIGURE 3 shows a simplified block diagram illustrating an
inventive method for generating low resolution "trick-play" data
streams for inclusion in pre-recorded digital records.
FIGURE 4 shows a simplified block diagram illustrating a
further inventive method for generating "trick-play" data streams
use
3 S for inclusion in pre-recorded digital records.
FIGURE 5 illustrates the derivation of predicted
macroblock DC coefficients.
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FIGURE 6 shows a simplified partial block diagram
illustrating a further inventive method for non-real-time generation
of pre-recorded records.
FIGURE 7 shows a simplified partial block diagram
illustrating another inventive method for non-real-time generation of
pre-recorded records.
DETAILED DESCRIPTION:
In a consumer digital video cassette recorder major
considerations in the real-time generation of a trick-play stream are
the complexity and cost of processing required, and the need to keep
this cost at a reasonable level. For this reason, the processing utilized
in the generation of a real-time trick-play data stream may be
limited to extracting pieces of the existing bit stream and
implementing minor modifications to bit-stream parameters.
"Trick-play" data streams must be produced in real-time
by extracting independent infra-information pieces from the original
data stream. This infra-information may come from infra-frames,
infra-slices, and/or infra-macroblocks. The source selected for I
frame data derivation depends on the form of infra refresh employed
2 0 in the original stream, and for exemplary purposes it is assumed that
either infra-frame or infra-slice refresh method is employed.
In a first inventive method of real-time generation, a low
spatial resolution "Trick Play" data stream is derived. The low spatial
resolution trick-play stream may, for example, have resolution
2 5 according to the CCIR 601 standard, (720 x 480 pixels), regardless of
the original HDTV stream resolution. Since the effective available bit-
rate for trick-play streams is limited to nominally 2 M. bits/sec.,
employing low spatial resolution in this manner results in fewer bits
being used per frame, and thus a relatively high temporal resolution
3 0 may be achieved. However, this low spatial resolution may only be
practical if an advanced television decoder and display is capable of
such resolution.
In a second inventive method a trick-play stream is
generated having the same resolution, or pixel count, as the original
3 5 HDTV material. However, since the usable trick-play bit-rate is
limited by the recording channel capacity of nominally 2 M. bits/sec.,
a trade-off exists between spatial and temporal resolution. Thus the
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provision of a full spatial resolution "Trick Play" mode effectively
requires that the temporal resolution be reduced to remain
commensurate with the TP data channel capacity.
The first inventive method for real-time generation of a
low spatial resolution "Trick Play" data is illustrated in FIGURE
1. In
r
this exemplary block diagram, trick-play speeds of Sx, 18x and
35x
are generated. For each TP speed, low-resolution, infra-coded
frames
are constructed from a received MPEG like transport stream.
By
detecting MPEG header information in the transport stream down
to
the slice level, infra slices can be extracted, processed and
used to
create a single I-frame in memory 110. The extraction and
processing stage 100 performs three tasks; extracting macroblocks
for
the construction of a TP I-frame, re-encoding DC transform
coefficients when necessary using DPCM encoding, and discarding
unwanted AC transform coefficients when necessary. Having
constructed and stored a low-resolution TP I-frame in memory
110, it
is utilized in the generation of speed specific data streams
for each
trick-play speed.
A radio frequency carrier, modulated responsive to an
2 0 MPEG compatible signal, is received by receiver O5. The modulated
carrier may be sourced from either an antenna or a cable, not
shown.
Receiver OS demodulates and processes the received carrier to
produce an MPEG compatible advanced television transport stream
09.
2 5 The advanced television transport stream 09, is
demultiplexed in block 20 to obtain only the Packetized Elemental
Stream or PES stream corresponding to the advanced television
video
information. The PES stream is decoded in block 30 to extract
from
the packets, the MPEG encoded video stream payload. Having
3 0 extracted the MPEG encoded stream, the required infra-coded
information may be detected and extracted. Sequence detection
block
40 examines the bit stream for the occurrence of a start code
characterized by twenty five 0's followed by 1, followed by
an 8 bit
address indicating MPEG video header. Picture detection is
3 5 performed in block 50 and in block 60 slice layers are detected.
Since an infra coded "trick-play" I frame is to be constructed
only
infra-slices are extracted. Infra-slices contain only infra-coded
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macroblocks, and are characterized by a 1-bit intra-slice flag in the
slice header. Thus when the intra-slice flag is set to 1 the entire slice
is passed to the "data extraction and processing" stage 100. The intra
detection process of .block 70 assumes that either intra-frame or
intra-slice refresh techniques are employed and also that the intra-
slice flag in the slice header is set when appropriate. If the
intra-slice flag is not set or intra-macroblock refresh is used then a
further level of detection down to macroblock level is required.
The data extraction and processing stage 100 selects from
the intra-coded macroblocks extracted in block 70, only intra
information which is utilized for constructing various trick-play data
streams. In addition block 100 performs any processing which may
be necessary to ensure the syntactic and semantic correctness for
MPEG compatibility of the resulting reconstructed TP I-frame. Since
the reconstructed TP I-frame is of lower spatial resolution than the
original MPEG stream, only a sub-set of the detected intra-
macroblocks is required. To determine which macroblocks or MBs are
to be kept and which are to be discarded, either a mathematical
function or a predefined look-up table may be employed. The
2 0 resulting lower spatial resolution frame results from the selected
patchwork of macroblocks. A controller stage 90 is coupled to
processing stage 100 and provides either, calculation required by the
mathematical function or provides the look up table for determining
macroblock selection.
2 5 The relationship between the MB position in the new low-
resolution I-frame,
(mb(i, j ), i=0, 1, 2, ... n-1, j = 0, 1, 2, ... m-1, where m and n are
the new I-frame width and height in MBs respectively and i and j
refer to the MB row and column) and the original full-resolution
3 0 frame ((MB(I, J), I=0, 1, 2, ... N-1, J=0, 1, 2, ... M-l, where M and N
are
the original frame width and height and I and J are the MB row and
column), the relationship is given by:
_ .
i (low-resolution row) - [L(n-1)/(N-1)]
3 S j (low-resolution column) - [J.(m-1 )/(M-1 )]
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where the product of the square brackets [x) denotes the integer
value closest to x.
The low resolution TP I frame utilizes a subset of the
macroblocks from the original frame with the remaining non-selected
MBs being discarded. FIGURE 5 illustrates an exemplary 4:2:0
sampled signal comprising three intra-coded macroblocks MB 1,
MB2
and MB3, where each comprises blocks 0, 1, 2, 3, 4 and 5. Macroblock
2 is crossed through to illustrate non-use in constructing the
reduced
resolution TP I frame. The DC coefficients of each luminance
and
chrominance block are depicted in FIGURE 5 with dark stripes.
The
DC coefficients are predicted from within each macroblock, with
the
DC coefficient of the first block of an MB being predicted from
the last
DC coefficient of the immediately preceding MB of the slice.
The
arrows in FIGURE 5 illustrate the prediction sequence. Thus,
if the
preceding MB, for example, MB 2 of FIGURE 5 is not selected,
certain
DC coefficients must be re-calculated from the newly abutted
macroblock, as depicted by arrows NEW of FIGURE 5, and re-encoded
using DPCM. This re-encoding process is performed as the
macroblocks are written to the I-frame memory 110.
2 0 If the HDTV video sequence originated from an interlaced
scanning source, an optional processing step may be included
to
remove interlace "flicker" exhibited by frozen interlaced fields
containing motion. If the temporal resolution of the reconstructed
trick-play stream is such that the same frame (two fields) is
2 5 displayed for more than one frame period, then such interlaced
"flicker" may be very noticeable. In field-coded macroblocks
this
"flicker" artifact may be eliminated by copying the top two
blocks of
the macroblock, blocks 0 and 1, to the lower two blocks, blocks
2 and
3. This copying within the macroblock effectively makes both
fields
3 0 the same thus removing any field-to-field motion from the frame.
This re-encoding process is performed as the macroblocks are
written
to the I-frame memory 110.
' A further function performed by processing stage 100 is
the removal of AC coefficients from each macroblock which cannot
be
3 5 accommodated in the newly constructed TP I-frame due to the
low
bit-rate available for the trick-play streams. To accomplish
this, each
block is variable-length-decoded to the point where the block
will be
WO 96/13121 .r, ~.~, ~ ~ ~ ~ PCT/US95/123=10
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padded with zeros, indicating the last coefficient of that block. The
number of bits for each block are stored and accumulate in a buffer.
The bits are counted and when a count exceeds a predetermined
number the remaining AC coefficients are unused or deleted. The
number of bits per TP MB depends on the overall rate allowed for
each trick-play stream and the temporal resolution or number of
frame updates per second.
The block diagram of FIGURE 1 illustrates the formation of
trick-play data streams having the same allocated bit-rate. If the
rate differs significantly between TP speeds, for example, to provide
differing resolution at each speed, then the number of AC coefficients
retained in I-frame memory 110 will also differ for each speed.
Hence I-frame memory 110 cannot be shared and separate I-frame
memories may be required for each TP speed or bit rate.
The inventive low-resolution TP I-frame assembled in I-
frame memory 110 is coupled to three trick-play stream generation
stages; 5 times, block 145; 18 times, block 160 and 35 times block
170. In exemplary FIGURE 1, each trick-play stream may be
allocated the same bit-rate and temporal resolution, which may
2 0 represent a preferred configuration. However, not every
reconstructed TP I-frame is used for each TP speed. For example, if
the I-frame refresh rate in the original stream is once every fifteen
frames (M=15) and the temporal resolution used by each trick-play
stream is selected to be three, i.e. the number of frame times between
2 5 frame updates, then for 5 times speed;
(5x speed). (3 frame repeats)/(15 frame refresh) = 1.0
thus every TP I-frame will be used. Similarly for 18x and 35x
3 0 speeds,
(18).(3)/(15) - 3.6 ~ '
(35).(3)/(15) - 7.0
3 5 Thus at 18x speed approximately every third or fourth I-frame is
used, and at 35x speed every seventh I-frame is used. If it is
assumed that the intra-refresh period in an advanced television
stream is 0.5 seconds (M=I5 for 30 fps source) then a three-frame
PCTlUS95/12340
WO 96!13121
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holding time for Sx speed is the highest possible TP temporal
_ resolution. For simplicity and consistency a three-frame holding
time
may be used for the remaining TP speeds. A higher temporal
resolution of two-frames or single-frame holding time could
be used
for higher TP speeds since lower temporal resolution at higher
speeds
may give a false sense of slower than actual trick-play speed.
Assuming that the effective trick-play bit-rate is constant,
the
provision of a higher temporal resolution would consequently
require
a lower spatial resolution quality.
The reconstructed TP I-frame is read from memory 110
and packaged, according to TP speed, by blocks 145, 160 and
170
which add the appropriate MPEG picture headers and a PES layer.
The advanced television transport stream 09 is buffered by buffer
15, which generates signal 10, a transport stream for normal
play
speed processing. Normal play transport stream 10 is coupled
to
multiplexor MUX 150. Multiplexor MUX 150 is controlled responsive
to recorder 210 servo signals to generate an output bit stream
having
a sequence which when recorded produces a predetermined track
format. The recorded track format is selected to provide the
desired
2 0 recorded TP bit rate and to facilitate specific physical location
of
speed specific TP I-frame packets within specific recorded tracks.
The recorded track format thus facilitates replay at normal
speed and
at the predetermined trick-play speeds. The TP I-frame packets,
Sx
signal 121, 18x signal 131 and 35x signal 141, are coupled to
2 5 multiplexor MUX 150 which inserts the I-frame packets for each
TP
speed into the normal play transport stream. Thus a valid, MPEG
like,
transport stream is formatted for record processing by recorder
210
and recording on tape 220.
To minimize TP bit rate, in place of repeated TP I frames,
3 0 frame repeats or holding times, may be implemented by writing
empty P-frames between I frames in the video stream. An empty
P-
frame results in the decoder predicting from the previous frame,
i.e.
- the TP I frame. Alternatively, frame repeats may be implemented
by
setting the DSM_trick_mode_flag in the PES layer and calculating
the
3 5 Presentation Time Stamp and Decode Time Stamp PTS/DTS values
such that each TP I frame is presented the necessary number
of frame
times apart. Either frame repeat method produces the same result.
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However, the second method requires no extra processing of the TP
stream on playback and hence, adds no extra cost to the unit.
However, the second method requires that the optional
DSM_trick_mode_flag is supported in advanced television decoders. ,
5 With this second method, the extra processing is implemented in the
advanced television decoder. Either frame repeat method may
implemented during speed specific stream generation in blocks 145,
160 and 170.
The inventive trick-play stream generation techniques
10 described above were employed to produce trick-play speeds of Sx,
18x and 35x with a spatial resolution of 720 x 480 pixels, and an
effective trick-play data rate of 2.0 Mbps. The various trick-play
speeds were evaluated and may be summarized by the following
points:
Data for each trick-play speed was generated representing
independent low-resolution (720 x 480 pixels), MPEG compatible
transport streams.
Each TP stream contains only intra-coded frames thus allowing
the same trick-play stream to be used for both Fast Forward and Fast
2 0 Reverse TP modes.
To retain a 16:9 aspect ratio, the actual spatial image size is
sampled to 720 x 384 pixels, with the remaining area above and
below the TP image black.
The temporal resolution is such that a constant three-frame
2 5 holding time is used resulting in an effective rate of 10 frames per
second.
Each I frame of the trick-play streams comprises a selection of
sampled macroblocks from the original stream. The bit rate of 2.0 M.
bits/sec. and three-frame holding time allows most AC coefficients to
3 0 remain in the selected macroblocks for typical test material.
The overall subjective spatial resolution is fair, being dependent
on the amount of motion and image complexity in the source material.
A picture rate of 10 fps provides good temporal resolution. The trick- '
play data stream may be decoded to produce recognizable trick-play
3 5 video images and hence is acceptable for tape search usage.
The inventive low-resolution real-time trick-play mode
previously discussed produces recognizable spatial images at a
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relatively high temporal resolution. However, as already mentioned,
this mode may be used if an advanced television receiver/decoder
unit is operable at lower resolution, for example, such as
that
produced by CCIR recommendation 601. However, if operation
at a
V
lower resolution is not provided, then trick-play data must
be
derived having nominally the same spatial resolution, i.e.
the same
pixel count as the original source. FIGURE 2 illustrates an
inventive
exemplary system for generating full-resolution, real-time
trick-play
streams. Three trick-play speeds of 5 times, 18 times and 35
times
are illustrated. The difference between the full-resolution
scheme of
FIGURE 2 and the low-resolution scheme illustrated in FIGURE
1, is in
data extraction and processing block 105, and stream generation
blocks 155, 165 and 175.
The transport stream decoding and infra detection
depicted in blocks 20, 30, 40, 50, 60, and 70 operate and function
as
described for the low resolution TP system of FIGURE 1. As
described
for the low resolution TP system, the purpose of the data extraction
and processing stage, block 105, is to extract only infra information
which is required for forming trick-play streams and to perform
any
2 0 processing which is required to guarantee the syntactic and
semantic
correctness of the resulting TP I-frame. The functionality
of block
105 differs from that of block 100 in that the regenerated
I-frame
must have the same resolution, or pixel count, as the original
data
stream. Hence, aII infra macroblocks are used to reconstruct
the new
2 5 TP I-frame. Since no MBs are deleted, no re-encoding of DC
transform
coefficients is required.
The major function of processing block 105 is the removal
of AC coefficients from each macroblock which, as a consequence
of
the trick-play bit-rate cannot be accommodated in the new TP
I-
3 0 frame. The low TP channel bit-rate, nominally 2 M. bit/sec.
forces a
trade-off between the number of AC coefficients used, i.e.
spatial
resolution, and the temporal resolution, or frame update rate
of the
trick-play stream. This spatial versus temporal trade-off was
also
present in the derivation of the low-resolution stream. However,
in a
3 5 full-resolution frame, i.e. same pixel count, the DC coefficients
alone
are likely to represent more bits than all the coefficients,
both AC and
DC assembled in a low-resolution TP frame. Thus any limited
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inclusion of even a few AC coefficients in each full-resolution
macroblock will produce a significant reduction in the temporal
resolution, i.e. the frame update time will be lengthened, with more
frame repeats. Thus to facilitate constant temporal resolution in full-
.5 resolution trick-play streams, a system may employ only the DC
coefficients of each macroblock with all AC coefficients being
discarded. In addition, discarding the AC coefficients reduces
processing complexity since only variable-length decoding of the
DPCM value of the DC coefficient is required. FIGURE 2 illustrates an
exemplary system where each trick-play speed has the same bit rate,
and thus the same I-frame memory may be shared between the
three TP speeds.
As discussed previously, if the original HDTV video images
were generated by interlaced scanning, then an optional processing
step may be included to remove interlace "flicker" exhibited by
frozen fields containing motion. One such method has already been
described. However, since this exemplary high resolution TP system
uses only DC transform coefficients, a simpler and more efficient
method may be provided by setting the frame_pred_frame_dct flag
2 0 in the picture_coding extension section to '1'. This flag indicates that
all MBs were frame encoded, thus a previously field-coded block,
which could produce 'flicker', is decoded as a frame-coded block. The
result is that each field is placed in either the upper or lower portion
of a block and any 'flicker' is removed. This method of flicker
2 5 elimination also reduces the number of bits used in the
macroblock_modes section since the dct type flag can no longer be
present if frame_pred_frame_dct_ is set to '1'.
The reconstructed TP I-frame is assembled in memory
115, and coupled to three trick-play stream generation stages, 5
3 0 times speed depicted in block 155, 18 times speed in block 165 and
35 times speed in block 175. The exemplary system of FIGURE 2
assumes that each trick-play stream has the same effective bit-rate
and hence the same approximate temporal resolution. As discussed -
previously, not every reconstructed TP I-frame is used for each
3 5 speed. However TP I-frame utilization may be further limited for the
following reason. Although each TP I-frame has the same number of
coefficients, for example, DC only, each TP I-frame may not have the
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same number of bits since the DC coefficients are variable length
encoded. Therefore, a constant temporal resolution or frame holding
_ time, cannot be fixed for each trick-play stream. Instead the frame
holding time will vary slightly over time with the number of bits
required to encode or form each TP I-frame. For each trick-play
speed, the respective "stream generation" stages, 155, 165 and 175,
wait until enough bits have been accumulated in buffer 105 to encode
a TP I-frame. Then if the TP I-frame accumulated in the buffer at
the time is a new TP I-frame, i.e. one which has not yet been encoded
in the specific trick-play speed, the TP I-frame is encoded and the
number of bits used will be subtracted from those available. If every
I-frame was the same size and each trick play speed was allocated
the same effective bit-rate, this scheme would be equivalent to that
described for the low-resolution system and the frame refresh period
would be constant for all speeds. The reconstructed TP I-frames are
read from memory 115 and packaged by stream generators 155, 165
and 175 to form a MPEG compatible transport streams in exactly the
same way as detailed for the low-resolution system.
The inventive full spatial resolution trick-play stream
2 0 generation technique described above was evaluated at an effective
trick-play data rate of 2.0 Mbps, for trick-play speeds of Sx, 18x and
35x. The performance may be summarized as follows:
An independent, TP I-frame-only MPEG compatible transport
stream may be recorded for each trick-play speed.
2 5 The temporal resolution varies with scene complexity and is
lower, having longer frame holding times than the low spatial
resolution trick-play system previously described. The average and
the variation in holding times experienced for typical source material
are shown in the following table:
TP SPEED AVERAGE HOLDING VARIATION IN
TIME IN FRAMES FRAMES
SX 5 FRAMES 5 - 8 FRAMES
18X 5 FRAMES S - 8 FRAMES
35X S FRAMES 5 - 8 FRAMES
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Note: Because an identical effective trick-play bit-rate is used
for all speeds, the temporal resolution will always be similar (if not _
identical) for each speed.
Each TP I-frame uses only DC coefficients. ,
The overall quality of spatial resolution is only fair since only
DC coefficients are used. The quality of temporal resolution may vary
between poor and fair, depending on the level of complexity within
the TP encoded material. However, the resulting trick-play images
are recognizable and acceptable for tape search usage.
The major differences between real-time trick-play and
pre-recorded trick-play data stream derivation, result from the
constraints of cost and lack of complexity imposed in a consumer
recorder/player. The consumer unit must derive and record the
trick-play data stream while recording normal replay data, i.e. the
trick-play data stream is derived in real-time. With pre-recorded
material, trick-play data streams may be derived directly from an
original picture source rather than from a compressed MPEG encoded
stream. Speed specific TP data streams may be derived
independently of one another and independently from the actual
2 0 recording event. Thus pre-recorded trick-play data may be derived
in non-real time, possibly at non-standard or slower frame repetition
rates. Since the constraints of the consumer real-time method no
longer apply, the quality of trick-play reproduction achieved by pre-
recorded material may be significantly higher.
2 5 A first inventive method of pre-recorded TP data
derivation provides a spatial resolution of for example, CCIR Rec. 601
having a resolution of 720 x 480 pixels, regardless of the original
HDTV stream resolution. A second inventive method constructs a
trick-play stream of the same resolution, i.e. pixel count, as the
3 0 original HDTV material.
FIGURE 3 illustrates an exemplary block diagram showing
an inventive method for generating low-resolution, pre-recorded
trick-play data streams. Regardless of the format of the original
HDTV video material 09, temporal processing block 30, performs
3 5 temporally subsampling which produces a 30 Hz, progressive signal
31. The operation of this stage may differ depending on whether the
original source material is progressive with a 59.94/60 Hz frame rate
WO 96113121 PCTIUS95/12340
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or interlaced with a 29.97/30 Hz frame rate. With progressively
scanned source material, the frame rate may be reduced by dropping
every second frame from the sequence. By dropping alternate
frames
a progressive sequence results having half the temporal resolution
of
5 the original source material. With interlaced source material,
the
frame rate remains the same but only one field from each frame
is
used. This processing results in a progressive sequence of half
the
vertical resolution and the same frame rate.
The progressively scanned frames, signal 31 is coupled to
10 block 40, which generates a lower resolution signal having,
for
example, the resolution delivered by CCIR Rec. 601. Each
Progressively scanned frame is resampled to 720 x 384 pixels
to
retain the 16:9 aspect ratio, and padded with black upper and
lower
borders to produce a 'letter-box' format of 720 x 480 pixels.
15 The HDTV signal is now represented by signal 41, having
a lower spatial resolution of 720 x 480 pixels, progressively
scanned
with a 30 Hz frame rate. Signal 41 is coupled to blocks 50,
60, 70
which implement speed-dependent temporal subsampling. Each
trick-play stream is constructed to have the same temporal resolution
2 0 or frame holding time of 2 frames, i.e. every frame will be
repeated
once. Therefore, at N times trick-play speed, the frame rate
is
reduced from 30 Hz to 30/2N Hz. Thus, the resulting recorded
frame
rates are as follows, Sx becomes 30/10 Hz, 18x becomes 30/36
Hz
and 35x becomes 30/70 Hz. Since every frame is presented twice
2 5 and the display rate is 30 Hz, the effective speed of scene
content
remains correct at each TP speed.
The temporal subsampling blocks 50, 60, 70, generate
output bit streams 51, 61 and 71 respectively, which are coupled
to
respective MPEG encoders 120, 130 and 140 to format MPEG
3 0 compatible bit streams. Since the MPEG compatible encoding is
the
same for each speed, and because in a pre-recording environment
real-time processing is not necessary, the same MPEG encoding
hardware may be used to encode the normal-play stream and each
trick-play stream. This commonalty of usage is indicated by
the
3 5 broken line enclosing the MPEG encoder blocks 100, 120, 130,
and
140. The temporally subsampled bit streams 51, 6I and 71 are
MPEG
encoded as I-frames. Each I-frame is repeated once by employing
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the DSM_trick_play_flag, located in the PES layer as described
previously. The resulting MPEG compatible streams representing
normal play speed NP, stream 101, and trick-play speeds of 5x,
stream 121, 18x, stream 131 and 35x, stream 141, are coupled for
record formatting by multiplexor 150. Multiplexor 150 effectively
selects between the various MPEG streams to generate a sync block
format signal 200, suitable for record processing by record replay
system 210 and writing to tape 220. As described earlier, the use of
predetermined TP speeds allows speed specific TP data to be
positioned, or recorded, at specific sync block locations within
recorded tracks. Thus multiplexor 150 formats sync block signal 200
to locate speed specific TP I frame data at specific sync block
locations within. the recorded tracks. These specific locations facilitate
reproduction at the various specific TP speeds.
FIGURE 6 is a partial block diagram illustrating a further
inventive arrangement of the non-real-time "trick-play" apparatus of
FIGURE 3. Speed specifically processed TP signals 51, 61 and 71 are
coupled to memories 520, 530 and 540 which store the 5 times, 18
times and 35 times processed digital image signals respectively. The
2 0 storage memories may be provided by means of disk or magnetic
tape recording and retrieval systems. The original HDTV signal 09 is
also stored in memory 500. Production of the prerecorded media or
tape is facilitated by the sequential selection between the various
stored digital signal sources to form an output signal which is MPEG
2 5 encoded by encoder 100 and recorded on the media. A multiplexor
150 is controlled to select between the various digital signal sources
to form an output signal for MPEG encoding. The MPEG encoded
signal 200 has the various signal components arranged such that a
recording may be replayed at normal and trick play speeds. Thus the
3 0 inventive arrangement of FIGURE 6 facilitates the non-real-time, and
independent derivation of both normal play and trick play digital
signal sources for encoding as MPEG compatible bit streams.
A further inventive arrangement is illustrated in block
600 which shows an alternative use for the multiplexed normal and
3 5 trick play MPEG encoded data stream produced by MUX 150. The
alternative system of block 600 replaces MPEG encoded data stream
200, recorder 210, format control signal FMT CTL, decoder 07 and
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display 300. In block 600, an MPEG encoded data stream 202
is
communicated to a transmitter 400 for coupling to decoder 07
and
display 300. A user viewing display 300 may chose to advance
the
material being viewed and initiates a remote control command
REM
CTRL which is communicated to multiplexor 150. Multiplexor
150
responds to the users remote command and selects, for example,
the
5 times speed bit stream 521 to be coupled for MPEG encoding
and
subsequent transmission, decoding and display. Similarly the
user
may chose to view in the reverse direction at, for example,
5 times
speed. This selection may be facilitated by reading the 5 times
play
speed memory 520 in reverse. Thus the arrangement of block
600
provides user controlled selection between normal and trick
play
MPEG encoded data streams.
FIGURE 7 is a partial block diagram illustrating another
inventive arrangement of the non-real-time "trick-play" apparatus
of
FIGURE 3. In FIGURE 7 both normal play and trick play processed
digital signals 09, 51, 61 and 71 are coupled for encoding
as MPEG
compatible bit streams by encoder 100. With non-real-time signal
processing and pre-recorded material preparation, signals 09,
51, 61
2 0 and 71 may be derived separately and individually coupled for
MPEG
encoding by a single encoder 100. The individually coded MPEG
bit
streams 101, 121, 131 and 141 are stored in memories 550, 560,
570
and 580 representing normal play and 5x, I8x and 35x bit streams
respectively. The storage memories may be provided by means
of
2 5 disk or magnetic tape recording and retrieval systems. Memories
550, 560, 570 and 580 produce output signals 501, 521 531 and
541
which are coupled to multiplexor 150 which is controlled responsive
to recorder 210 to generate an MPEG compatible record bit stream
formatted such as to provide reproduction at normal play speed
and
3 0 at the predetermined "trick-play" speeds.
A further inventive arrangement is illustrated in block
600 which shows an alternative use for the multiplexed normal
and
trick play 1VIPEG encoded data stream produced by MUX 150.
The
alternative system of block 600 replaces MPEG encoded data
stream
3 5 200, recorder 210, format control signal FMT CTL, decoder 07
and
display 300. In block 600, an MPEG encoded data stream 202
is
communicated to a transmitter 400 for coupling to decoder 07
and
PCT/US95/12340
WO 96/13121
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display 300. A user viewing display 300 may chose to advance the
material being viewed and initiates a remote control command REM
CTRL which is communicated to multiplexor 150. Multiplexor 150
responds to the users remote command and selects, for example, the
5 times speed bit stream 521 to be coupled for MPEG encoding and
subsequent transmission, decoding and display. Similarly the user
may chose to view in the reverse direction at, for example, 5 times
speed. This selection may be facilitated by reading the 5 times play
speed memory 520 in reverse. Thus the arrangement of block 600
provides user controlled selection between normal and trick play
MPEG encoded data streams.
The exemplary, low spatial resolution TP system
illustrated in FIGURE 3, and described above, produces trick-play
quality significantly higher than that attainable from real-time
derived trick-play streams. The results produced may be
summarized as follows.
During recording, an independent, I-frame only, low-resolution
(720 x 480 pixel) MPEG compatible stream is written to tape for each
trick-play speed.
2 0 The actual spatial image size is 720 x 384 pixels, to retain 16:9
aspect ratio, presented in a "letter box" format.
The temporal resolution is effectively 15 frames/second for
each trick-play speed and produces good to excellent quality which
remains constant for each speed.
2 5 The spatial resolution produced by a 2.0 Mbps data rate and _
720 x 480 pixels resolution is good to very good, depending on the
complexity of the source material.
Overall, the trick-play image quality exhibited with this scheme
is very high.
3 0 The low-resolution pre-recorded trick-play system shown
in FIGURE 3 and describe above produces good quality spatial images '
at a relatively high temporal resolution. However, such a low-
resolution method may be used providing the advanced television
decoder/receiver unit is able support the lower resolution display
3 5 format.
FIGURE 4 is an exemplary block diagram of an inventive
full-resolution, pre-recorded trick-play stream generation system,
WO 96/13121 PCTlUS95/12340
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19
providing trick-play speeds of, Sx, 18x and 35x. As previously
discussed, pre-recorded trick play data stream derivation may
be
generated from the original, uncompressed, source material.
FIGURE
4 illustrates the generation of normal-play and trick-play bit
streams,
however these ma be
y generated independently of one another,
directly from the HDTV source material. Since this system provides
full-resolution, no spatial sub-sampling is required and hence
less
processing is required than that shown in FIGURE 3. Since the
original, uncompressed, source material may be used, frames
which
1 0 are to be intra-coded may be chosen exactly to suit the trick
play
speed, rather than selecting I frames from an encoded stream.
In
addition a constant temporal refresh rate can be maintained,
which is
more pleasing to the user.
The original HDTV video signal 09 is shown coupled to
MPEG encoder 100 which generates an MPEG stream 101 for normal
play speed operation. Signal 09 is also coupled for temporal
subsampling in blocks 55, 65 and 75 respectively. For a trick-play
speed of N times, only every Nth source frame may be utilized
for
coding. However, depending on a desired trade-off between spatial
2 0 and temporal resolution, the actual frames used for encoding
may be
closer to every SNth or BNth frame in order to provide an acceptable
spatial resolution. Hence frame holding times, or temporal resolution,
are similar to those of the real-time, full-resolution system
described
earlier.
2 5 Having selected a frame holding or update time, for
example, every SNth frame for each N times trick-play speed
the
HDTV stream, signal 09, is temporally sub-sampled for each TP
speed.
The 5 times TP stream is derived in block SS which temporally
sub-
samples by a factor of 1/5N, or 1/25, i.e. 1 frame in 25 is
selected to
3 0 generate output signal 56. Similarly, the 18 times TP stream
is
derived in block 65, which temporally sub-samples by a factor
of
1/5N, or 1/90 and generates output signal 66. The 35 times TP
stream is derived in block 75, which temporally sub-samples
by a
factor of 1/5N, or 1/175 and generates output signal 76. The
three
3 5 sub-sampled TP bit stream signals, 56, 66 and 76 are coupled
for
MPEG encoding in encoder blocks 120, 130 and 140 respectively.
WO 96113121 PCT/US95/12340
2 Z 0-1~ 71 1
Since MPEG compatible encoding is the same for each
speed, and because real-time processing is not necessary in a pre-
recording environment, the same MPEG encoding hardware may be
used to encode the normal-play stream and each trick-play stream.
5 This commonalty of usage is indicated by the broken line enclosing
the MPEG encoder blocks 100, 120, 130, and 140. The temporally
subsampled bit streams 56, 66 and 76 are MPEG encoded as I-frames.
Because the frame update time is constant throughout each trick-play
stream, so is the number of bits allocated for each I-frame. The
10 frame holding times, or I-frame repeats may implemented by
employing the DSM_trick_play_flag as previously described. The
resulting MPEG transport streams representing normal play speed NP,
stream 101, and trick-play speeds of 5x, stream 121, 18x, stream 131
and 35x, stream 141, are coupled for record formatting by
15 multiplexor 150. Multiplexor 150 effectively selects between the
various MPEG streams to generate a sync block format signal 200,
suitable for record processing by record replay system 210 and
writing to tape 220. As previously described, predetermined TP
speeds allow speed specific TP data to be positioned, or recorded, at
2 0 specific locations within recorded tracks. Thus multiplexor 150
formats sync block signal 200 to locate speed specific TP I frame data
at specific sync block locations which facilitate reproduction at the
various specific TP speeds.
The inventive arrangements of FIGURES 6 and 7 may also
2 5 be applied the non-real-time "trick-play" generation arrangement of
FIGURE 4. As has been described, the arrangements of FIGURES 6 and
7 may facilitate non-real-time, and independent derivation of normal
play and trick play digital signals, and the encoding of MPEG
compatible bit streams.
3 0 The storage and retrieval memories 550, 560, 570 and
580 of FIGURE 7, generate MPEG compatible output signals 501, 521 -
531 and 541 which are illustrated coupled via multiplexor MUX 150.
The the MPEG output stream 200 generated by MUX 150 may, in an
alternative inventive embodiment, be coupled to a transmission
3 5 system for distribution for user display. Multiplexor MUX 150 may
be controlled to select between memory derived MPEG compatible
output signals responsive to user commands. For example, a user
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21
viewing a program at normal speed will receive and decode bit
stream signal 202. The user may desire to advance or fast forward,
the program by remotely selecting viewing of for example the five
time speed MPEG bit stream. The user remote command for Sx
replay, causes multiplexor 150 to select MPEG bit stream 521 which is
output to the user.
The constraints of retaining full spatial and temporal
resolution, result in a trick-play quality which is very similar to that
achieved by the full-resolution real-time method. However, this pre-
recording method has an advantage that the frame holding time is
constant. The trick-play stream generation technique described
provides trick-play speeds of Sx, 18x and 35x, having full spatial
resolution, and an effective trick-play bit rate of 2.0 Mbps. The
performance may be summarized as follows:
During recording, an independent, I-frame only, MPEG stream is
written to tape for each trick-play speed.
The spatial resolution is the same as the source material.
The temporal resolution is fixed having a 5 frame holding time.
Each I-frame uses all DC and some AC coefficients.
2 0 The overall spatial quality is fair. Recovered trick-play images
are recognizable and are acceptable for tape search purposes.
The following table summarizes trick-play quality
achieved by the various inventive methods disclosed.
' WO 96/13121 PCT/US95/12340
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22
REAL-TIME TRICK NON-REAL-TIME
PLAY STREAM TRICK PLAY STREAM
GENERATION GENERATION
FULL SPATIAL (ZUALITY: SPATIAL (2UALITY:
RESOLUTION poor to fair, only poor to fair, DC
DC &
TRICK PLAY coefficients used. some AC coefficients
MODES used.
TEMPORAL QUALITY: TEMPORAL QUALITY:
poor to acceptable, poor to acceptable,
variable 5-8 frame constant S frame
holdin times holdin time.
LOW SPATIAL QUALITY: SPATIAL (,QUALITY:
RESOLUTION poor to good, dependsgood to very good,
TRICK PLAY on material, depends on material,
MODES patchwork of MBs
used.
TEMPORAL (,QUALITY: TEMPORAL QUALITY:
good, constant 3 very good, constant
2
frame holdin time. frame holdin time.
In view of the constraints discussed previously, the highest trick-play
quality may be achieved, in both real-time and pre-recorded
S material, by the use of lower-resolution trick-play data. However,
the advanced television receiver/decoder must support the use of a
low resolution mode. If full-resolution trick-play modes are utilized,
the quality provided may be enhanced by manipulation of various
parameters. For example, raising the effective bit-rate available for
each trick-play speed, will allow an increase in resolution. However,
a minimum bit-rate of approximately 2.0 Mbps is required. If the
number of "Trick Play" speeds provided is reduced, for example to
two in each direction, then the effective bit-rate for each remaining
speed may be increased. The effective temporal resolution, or
number of frame repeats, results from the trade-off between
temporal and spatial resolution. Hence either parameter may be
optimized depending on the desired application.
